Known in the art are so-called “MONOS” (metal oxide nitride oxide semiconductor) type and other non-volatile semiconductor memory devices having charge storage films comprised of a stacked plurality of dielectric films and storing information by controlling amounts of charges stored in charge traps in the charge storage films.
Recently, technology taking note of the fact that it is possible to inject a charge into part of a region of distribution of dispersed charge traps by the conventional CHE (channel hot electron) injection method and independently write binary information at a source side and a drain side of a charge storage film so as to enable independent storage of two bits of information in one memory cell has been reported.
For example, in “2000 Symposium on VLSI Technology, pp. 122-123”, charge storage films are separately provided on the source side and the drain side, control electrodes are provided on the charge storage films, and word gate electrodes are provided at a center portion of a channel between the control electrodes in a state with a single-layer dielectric film not having a charge holding ability interposed. The word gate electrodes are connected to a word line, while the control electrodes are laid in a direction orthogonal to the word line and are controlled separately from the word gate electrodes. Therefore, the controllability of a position of charge injection and the charge injection efficiency can be improved and, as a result, a high speed write operation is attained.
The memory cells, which are called “twin MONOS cells”, have word gate electrodes repeated at a certain interval in the row direction, and have sidewall type conductive layers on wall surfaces on the two sides of the same in the row direction. ONO (oxide-nitride-oxide) films, namely, charge storage films having a charge holding ability, are provided directly under the sidewall type conductive layers. As opposed to this, single layers of dielectric films are formed directly under the word gate electrodes, thus these parts do not have any charge holding ability.
The sidewall type conductive layers and the word gate electrodes are used as masks for introducing N-type impurities to substrate locations exposed between adjacent sidewall type conductive layers to form N+-type impurity regions serving as sources or drains.
The above paper did not disclose a specific production method, but a twin MONOS cell has the following problems in production and structure.
In twin MONOS cells, the word gate electrodes are formed, then sidewall type conductive layers are formed at their sides. Thus, a step of connecting the word gate electrodes to the word line is necessary.
Further, the word gate electrodes in twin MONOS cells first need to be patterned to parallel line shapes long in the column direction. At this time, normally, the word gate electrode material is deposited, then patterns of a resist are formed on it and the word gate electrode material is processed by an etching method having strong anisotropy, for example, by RIE (reactive ion etching), using the resist as a mask. The resist patterns normally are tapered forward in sectional shapes at their side surfaces, and the resist recesses somewhat at the time of etching, so the sides of the word gate electrodes after the processing also are tapered forward. Further, even when a resist is not used and a material which does not recess at the time of etching is used, forward tapering is liable to arise to a certain extent at the side surfaces of the word gate electrodes after the processing due to the effect of sidewall deposits at the time of etching.
The word gate electrodes for example need to be simultaneously processed when patterning the word lines to isolate them for cells. However, at this time, since the control gates are already formed at the sidewalls of the word gate electrodes in a state having insulation films interposed, the word gate electrodes have to be selectively etched and removed while digging holes having trapezoidal sectional shapes. Accordingly, at the time of this etching, the bottom sides of the side surfaces of the reverse tapered control electrodes are difficult to etch and conductive residue easily is produced in these portions along the control electrodes. If conductive residue is produced, short-circuit occur between the word lines.
Further, the sidewall type conductive layers are formed in ring shapes circling the periphery of the line shaped conductive layers for forming the word gate electrodes. If the sidewall type conductive layers are used for the control electrodes as they are, a control electrode on the source side and a control electrode on the drain side would become electrically short-circuited. Therefore, to supply different voltages to the control electrode on the source side and the control electrode on the drain side, the two control electrodes have to be isolated. This isolation cannot be performed all together in another step, for example, at the time of processing the word lines, so for example a step of forming an etching mask opened only at the two end portions of the line shaped conductive layers for forming the word gate electrodes, removing an insulation film covering the sidewall type conductive layers through the openings, and cutting the conductive layers by etching becomes necessary.
Furthermore, in a twin MONOS cell, since ONO films are formed directly under the sidewall type conductive layers, the ONO films contacting the channel formation region extend long in the column direction along the sidewall type conductive layers. During operation, data is written by injecting charges to a region of an ONO film crossing with a channel (hereinafter referred to as a storage), while data is erased by draining the stored charge to the substrate side or by injecting a reverse conductivity type charge. When repeating this rewrite operation, a charge is liable to accumulate continuously in an adjacent region of the storage. Further, a leak path to the outside of the channel is liable to be formed by the charge. When erasing data by draining the stored charges, which are electrons, from the entire surface of the channel, since the adjacent region is also under control of a control electrode in the same way as the storage and electrons accumulated at the adjacent region are also drained at the same time, this does not become much of a problem. However, particularly when injecting a charge having a reverse polarity to a storage for erasing stored charge, if a charge having a polarity of a direction turning on the channel, for example, an electron hole in the case of an N-type channel, is accumulated in an adjacent region of the storage, a leak path will be easily formed. The resultant decline of the leak characteristic therefore becomes a problem.